Acute lung injury (ALI) is frequently associated with trauma and blood loss. In ALl, macrophages and neutrophils express increased levels of proinflammatory cytokines and chemokines, and demonstrate enhanced activity of the transcriptional regulatory factor NF-kB. High mobility group box 1 (HMGB1) protein, originally identified as a DNA binding protein, was recently found to have potent proinflammatory properties, including the ability to activate neutrophils and macrophages to express increased levels of proinflammatory cytokines. HMGB1 is a late mediator of endotoxin lethality and LPS induced ALl, and can itself produce ALl. In patients with severe sepsis, elevated serum HMGB1 levels are associated with increased mortality. Although circulating HMGB1 levels are also elevated after severe hemorrhage, the role of HMGB1 in hemorrhage and trauma induced ALl is presently unknown. Characterization of the molecular mechanisms, receptors, and transcriptional regulatory mechanisms through which HMGB1 enhances neutrophil and macrophage activation and contributes to the development of ALl after hemorrhage and trauma are the major goals of the proposed studies. Our hypothesis is that HMGB1 contributes to the development of acute lung injury after hemorrhage and trauma. The Specific Aims of this project are: 1) To investigate the effects of hemorrhage on circulating and pulmonary HMGB1 expression, as well as the role of HMGB1 in hemorrhage induced ALl;2) To determine the receptors and signaling pathways involved in HMGB1 induced cellular activation and the development of ALl;and 3) To examine in conjunction with the Clinical Core the relationship between circulating and pulmonary HMGB1 levels and the mechanism and sites of trauma as well as outcome from trauma. The proposed experiments should provide novel insights into the mechanisms involved in cellular activation and the development of acute lung injury after severe blood loss and injury. In addition, results from these studies are likely to suggest new therapeutic approaches aimed at improving outcome from important clinical conditions, such as trauma and hemorrhage associated mortality and acute lung injury, in which HMGB1 plays a major role.